Alcohol based method of making multi-walled carbon nanotube nanocomposite

ABSTRACT

A method for the preparation of zirconia-multi-walled carbon nanotube nanocomposite utilizing Pluronics as templating agents is described. An efficient method for producing hydrogen gas using the nanocomposite as a photocatalyst.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to zirconia-multi-walled carbonnanocomposite, a method of manufacturing a zirconia multi walled carbonnanotube (MWCNT) composite and the use of a zirconia-multi-walled carbonnanocomposite as a photocatalyst for the production hydrogen from water.

Description of the Related Art

Zirconia (ZrO₂) has long been used in many applications. It hasfavorable mechanical properties such as toughness, hardness, impactstrength and ionic properties, and is utilized in many applications suchas structural reinforcements, dental implants, and sensors. Thecatalytic activity of zirconia has been utilized in numerous reactions[Srinivasan et al. “Zirconia: a review of a super ceramic”, in: D. L.Perry (Ed.), Materials Synthesis and Characterization, Springer, US,1997, pp. 147-188; and Almeida et al. “Enhanced mechanical properties inZrO₂ multi-walled carbon nanotube nanocomposites produced by sol-gel andhigh-pressure” Nano-Structures & Nano-Objects, 4 (2015) 1-8]. With theadvancement of nanomaterial synthesis, new approaches for the synthesisof zirconia based materials enabled tailoring the material for aspecific purpose by enhancing certain desired properties. For example,Almeida et al. used the sol-gel approach to enhance the mechanicalproperties of ZrO₂-multi-walled carbon nanotube nanocomposite. Amin etal. [“Electrocatalytic activity of PteZrO₂ supported on different carbonmaterials for methanol oxidation in H₂50₄ solution” InternationalJournal of Hydrogen Energy 41 (2016) 1846-1858] describe the synthesisand electrolytic properties of Pt—ZrO₂/MWCNTs and other compounds formethanol oxidation in sulfuric acid solution by a solid state reactionmethod utilizing intermittent microwave heating. Malolepszy et al.[“Deactivation resistant PdZrO₂ supported on multiwall carbon nanotubescatalyst for direct formic acid fuel cells” International Journal ofHydrogen Energy 40 (2015) 16724-16733] utilized a microwave assistedhydrothermal method for the synthesis of Pd-ZrO₂/MWCNT and used it toconstruct a formic acid fuel cell. Another fuel cell was built using aPt catalyst supported on sulfated MWCNT-ZrO₂. In the preparation of thenanocomposite, ammonia and sulfuric acid were used as precipitating andsulfating agents, respectively [Dao-Jun et al. “Synthesis of sulfatedZrO₂/MWCNT composites as new supports of Pt catalysts for directmethanol fuel cell application” Applied Catalysis B: Environmental 89(2009) 597-601]. Michalek et al. [Mechanical and functional propertiesof Al₂O₃—ZrO₂-MWCNTs nanocomposites, Journal of the European CeramicSociety 34 (2014) 3329-3337] describes the mechanical and functionalproperties of Al₂O₃—ZrO₂-MWCNT nanocomposites synthesized by a methodthat included the use of liquid nitrogen and freeze drying. A cobaltdoped ZrO₂ decorated multi-walled carbon nanotube catalyst wassynthesized via a homogenous co-precipitation process and was utilizedfor the photo-degradation of indigo, carmine, and eosin Y dyes [Anku etal. “Cobalt doped ZrO₂ decorated multi-walled carbon nanotube: Apromising nanocatalyst for photo-degradation of indigo carmine and eosinY dyes” Progress in Natural Science: Materials International 26 (2016)354-361]. Anku et al. describes the synthesis ofpalladium-doped-ZrO₂-MWCNTs nanocomposite employing a homogenousco-precipitation method and used the nanocomposite in water treatment[Palladium-doped-ZrO₂-multiwalled carbon nanotubes nanocomposite: anadvanced photocatalyst for water treatment, Appl. Phys. A (2016)122:579]. Wang et al. [Fabrication and characterization of azirconia/multi-walled carbon nanotube mesoporous composite, MaterialsScience and Engineering C 33 (2013) 3931-3934] reported the synthesis ofa mesoporous MWCNT-ZrO₂ nanocomposite via a hydrothermal method and theuse of the cationic surfactant cetyltrimethylammonium bromide. Liu etal. [Controlling the Particle Size of ZrO₂ Nanoparticles inHydrothermally Stable ZrO₂/MWCNT Composites, Langmuir 2012, 28,17159-17167] describes a method of controlling particles size of ZrO₂ ina composite of MWCNTs decorated with ZrO₂ nanoparticles by a graftingmethod followed by high-temperature annealing.

The present disclosure describes the synthesis of a ZrO₂-multi-walledcarbon nanotube nanocomposite (MWCNT-ZrO₂) using a templating agent. Themethod may be carried out using available laboratory equipment. Thenanocomposite produced by the method is an efficient photocatalyst andcan be utilized in a method for the production of hydrogen.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the invention, the current disclosurerelates to a method of making a ZrO₂-multi-walled carbon nanotubenanocomposite (MWCNT-ZrO₂), comprising:

-   -   dispersing zirconium alkoxide and a templating agent in an        alcohol to form a mixture, wherein the ratio of the zirconium        alkoxide to the alcohol to the templating agent is in the range        of 0.2:10:0.1 to 3:10:0.5,    -   stirring the mixture at a temperature in the range of 40-80° C.,    -   mixing MWCNT with the mixture, and    -   mixing an aqueous carboxylic acid solution with the        MWCNT-containing mixture to precipitate the multi-walled carbon        nanotube-ZrO₂ nanocomposite.

In a preferred embodiment of the method, the templating agent is anon-ionic surfactant, preferably a poloxamer such as but not limited toPluronic F-127, Pluronic P-123, Pluronic F-108, and Pluronic F-68. In amore preferred embodiment, the templating agent is Pluronic F127.

In another preferred embodiment, the zirconium alkoxide is zirconiummethoxide, zirconium ethoxide, zirconium propoxide, zirconiumisopropoxide, zirconium butoxide, zirconium isobutoxide, and zirconiumt-butoxide. In a preferred embodiment, the zirconium alkoxide iszirconium isopropoxide.

In another preferred embodiment, the solvent is an alcohol, preferablymethyl alcohol, ethyl alcohol, propyl alcohol, isobutyl alcohol, butylalcohol, isobutyl alcohol, and t-butyl alcohol, and most preferablyisopropyl alcohol.

In another preferred embodiment, the ratio of zirconium alkoxide tosolvent to the templating agent is in the range of 0.2:10:0.1 to3:10:0.5, preferably 0.5:10:0.2 to 1.5:10:0.4, more preferably0.8:10:0:25. In the most preferred embodiment of the method, the ratiois 1:10:0.3.

In another embodiment, the produced MWCNT-ZO₂ nanoparticle comprisesMWCNT in the range 0.5%-8% by weight, preferably 0.7%-7% by weight ofMWCNT, more preferably 0.8%-6% by weight of MWCNT, and even morepreferably 1.0%-5.0% by weight of MWCNT. In the most preferredembodiment of the method, the produced MWCNT-ZO₂ nanoparticle comprises3% by weight of MWCNT.

Any acid may be used to precipitate the nanoparticles in any of thementioned embodiments of the method. The preferred acid is a carboxylicacid such as acetic acid, propionic acid, butyric acid, oxalic acid,succinic acid, citric acid, and tartaric acid. In the most preferredembodiment of the method, acetic acid may be used.

In another preferred embodiment, the mixture is stirred at a temperaturein the range of 20-80° C., preferably 30-70° C., more preferably, 40-65°C., and most preferably 50-60° C.

In some embodiments, the MWCNT-ZrO₂ is filtered and dried under vacuumat about 60° C.

In some other preferred embodiment, the dried MWCNT-ZrO₂ is calcined ofat about 300° C. under nitrogen.

A second aspect of the invention is related to a MWCNT-ZrO₂nanocomposite, preferably prepared by the method described herein.

A preferred embodiment of the nanocomposite contains about 0.5-8% MWCNTby weight of MWCNT, preferably 0.7%-7%, more preferably 0.8%-6% byweight of MWCNT, and even more preferably 1.0%-5.0% by weight of MWCNT.In the most preferred embodiment of the method, the produced ZO₂-MWCNTnanoparticle comprises 3% by weight of MWCNT.

In another preferred embodiment, the nanocomposite has a surface area inthe range of 53.5±2 to 59.5±2 m²/g. A more preferred embodiment of themethod, the nanocomposite has a surface area in the range of 56.5±1 to59.5±1 m²/g. The most preferred embodiment, the nanocomposite has asurface area of 59.2±0.6 m²/g.

In another preferred embodiment, the nanocomposite is a photocatalysthaving band gap energy in the range 3.33 to 2.63 eV. In a more preferredembodiment, the band gap is 2.67±0.05.

In another preferred embodiment, the nanocomposite contains MWCNT in anamount selected from the group consisting of 1 wt. %, 2 wt. %, 3 wt. %,4 wt. %, and 5 wt. %. In the most preferred embodiment, thenanocomposite contains 3 wt. % MWCNT.

A third aspect of the invention is directed to a photochemical methodfor the production of hydrogen from water, the method comprisingirradiating a reaction mixture comprising an aqueous solution and aphotocatalyst comprising the MWCNT-ZrO₂ nanocomposite with light to formhydrogen gas.

In a preferred embodiment, the photocatalyst is selected from the groupconsisting of 2 wt. % MWCNT-ZrO₂, 3 wt. % MWCNT-ZrO₂, 4 wt. %MWCNT-ZrO₂, and 5 wt. % MWCNT-ZrO₂.

In another preferred embodiment, the reaction mixture contains about0.5-4.0 g/L photocatalyst, more preferably 1.0-3.0 g/L, and mostpreferably, 1.5-2.5 g/L.

In another preferred embodiment, the reaction mixture contains about2.0±0.2 g/L of the photocatalyst.

In another preferred embodiment, reaction mixture comprises an alcoholwherein the alcohol is selected from the group consisting of methanol,ethanol, propanol, isopropanol, butanol, isobutanol, and t-butanol.

In a preferred embodiment, the reaction mixture contains alcohol in therange 2%-20%, preferably, 5%-15%, more preferably 8%-12%, and mostpreferably 9%-11%.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 XRD patterns of MWCNT, ZrO₂, and MWCNT-ZrO₂ nanocomposite having1, 2, 3, 4, and 5 wt. % of MWCNT.

FIG. 2A TEM image of ZrO₂.

FIG. 2B TEM image of MWCNT.

FIG. 2C TEM image of 1 wt. % MWCNT-ZrO₂.

FIG. 2D TEM image of 2 wt. % MWCNT-ZrO₂.

FIG. 2E TEM image of 3 wt. % MWCNT-ZrO₂.

FIG. 2F TEM image of 4 wt. % MWCNT-ZrO₂.

FIG. 2G TEM image of 5 wt. % MWCNT-ZrO₂.

FIG. 3 UV-Vis spectra of ZrO₂, MWCNT-ZrO₂.

FIG. 4 PL spectra of ZrO₂, MWCNT-ZrO₂, and 1-5 wt. % MWCNT-ZrO₂nanocomposites.

FIG. 5 Transient photocurrent responses of ZrO₂ and 3 wt. % MWCNT-ZrO₂photocataly st

FIG. 6 Effect of type of photocatalyst on amount of hydrogen evolution.

FIG. 7 Effect of 1, 2, 3, 4, 3, and 5 wt. % MWCNT-ZrO₂ photocatalyst onthe amount of hydrogen evolution.

FIG. 8 Effect of recycling 3 wt. % MWCNT-ZrO₂ photocatalyst on theamount of hydrogen produced.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all embodiments of the disclosure are shown. The presentdisclosure will be better understood with reference to the followingdefinitions.

All publications mentioned herein are incorporated herein by referencein full for the purpose of describing and disclosing the methodologies,which are described in the publications, which might be used inconnection with the description herein. The publications discussed aboveand throughout the text are provided solely for their disclosure priorto the filing date of the present application. Nothing herein is to beconstrued as an admission that the inventors are not entitled toantedate such disclosure by virtue of prior disclosure. Also, the use of“or” means “and/or” unless stated otherwise. Similarly, “comprise,”“comprises,” “comprising” “include,” “includes,” and “including” areinterchangeable and not intended to be limiting.

As used herein, the terms “compound” is intended to refer to a chemicalentity, whether in a solid, liquid or gaseous phase, and whether in acrude mixture or purified and isolated.

As used herein, the term “alkyl” unless otherwise specified refers toboth branched and straight chain saturated aliphatic primary, secondary,and/or tertiary hydrocarbons of typically C₁ to C₁₀, and specificallyincludes, but is not limited to, methyl, trifluoromethyl, ethyl, propyl,isopropyl, cyclopropyl, butyl, isobutyl, t-butyl, pentyl, cyclopentyl,isopentyl, neopentyl, hexyl, isohexyl, cyclohexyl, cyclohexylmethyl,3-methylpentyl, 2,2-dimethylbutyl, and 2,3-dimethylbutyl. As usedherein, the term optionally includes substituted alkyl groups. Exemplarymoieties with which the alkyl group can be substituted may be selectedfrom the group including, but not limited to, hydroxyl, alkoxy, aryloxy,or combination thereof.

As used herein, the term “alcohol” unless otherwise specified refers toa chemical compound having an alkyl group bonded to a hydroxyl group.Many alcohols are known in the art including, but not limited to,methanol, ethanol, propanol, isopropanol, butanol, isobutanol andt-butanol, as well as pentanol, hexanol, heptanol and isomers thereof.Since the alkyl group may be substituted with one or more hydroxylgroup, the term “alcohol” includes diols, triol, and sugar alcohols suchas, but not limited to, ethylene glycol, propylene glycol, glycerol, andpolyol.

As used herein, the term “about” refers to an approximate number within20% of a stated value, preferably within 15% of a stated value, morepreferably within 10% of a stated value, and most preferably within 5%of a stated value. For example, if a stated value is about 8.0, thevalue may vary in the range of 8±1.6, ±1.0, ±0.8, ±0.5, ±0.4, ±0.3,±0.2, or ±0.1.

The present disclosure is further intended to include all isotopes ofatoms occurring in the present compounds. Isotopes include those atomshaving the same atomic number but different mass numbers. By way ofgeneral example, and without limitation, isotopes of hydrogen includedeuterium and tritium. Isotopes of carbon include ¹³C and ¹⁴C. Isotopesof zirconium include ⁹⁰Zr, ⁹¹Zr; ⁹²Zr, ⁹³Zr, ⁹⁴Zr, and ⁹⁶Zr. Isotopes ofoxygen include ¹⁶O, ¹⁷O, and ¹⁸O. Isotopically labeled compounds of theinvention can generally be prepared by conventional techniques known tothose skilled in the art or by processes and methods analogous to thosedescribed herein, using an appropriate isotopically labeled reagent inplace of the non-labeled reagent otherwise employed.

As used herein a “polymer” or “polymeric resin” refers to a largemolecule or macromolecule, of many repeating subunits and/or substancescomposed of macromolecules. As used herein a “monomer” refers to amolecule or compound that may bind chemically to other molecules to forma polymer. As used herein the term “repeat unit” or “repeating unit”refers to a part of the polymer or resin whose repetition would producethe complete polymer chain (excluding the end groups) by linking therepeating units together successively along the chain. The process bywhich monomers combine end to end to form a polymer is referred toherein as “polymerization” or “polycondensation”, monomers are moleculeswhich can undergo polymerization, thereby contributing constitutionalrepeating units to the structures of a macromolecule or polymer. As usedherein “resin” or “polymeric resin” refers to a solid or highly viscoussubstance or polymeric macromolecule containing polymers, preferablywith reactive groups. As used herein a “copolymer” refers to a polymerderived from more than one species of monomer and are obtained by“copolymerization” of more than one species of monomer. Copolymersobtained by copolymerization of two monomer species may be termedbipolymers, those obtained from three monomers may be termed terpolymersand those obtained from four monomers may be termed quarterpolymers,etc. As used herein, “cross-linking”, “cross-linked” or a “cross-link”refers to polymers and resins containing branches that connect polymerchains via bonds that link one polymer chain to another. The cross-linkmay be an atom, a group of atoms, or a number of branch points connectedby bonds, groups of atoms, or polymer chains. In the majority of cases,a cross-link is a covalent structure or covalent bond but the term mayalso describe sites of weaker chemical interactions, portioncrystallites, and even physical interactions and entanglements. Thecross-linking can alter the physical and mechanical properties of thepolymer. Cross-linking may be formed by chemical reactions that areinitiated by heat, pressure, change in pH, and/or radiation, with orwithout the presence of a cross-linking agent and/or catalyst. Incertain embodiments, at least one diaminoalkane or di-dithiocarbamatealkane functions as a cross-linking agent for the cross-linked polymericresin described herein. In a preferred embodiment, the diaminoalkane ordi-dithiocarbamate chains function as cross-linking agents or monomerscan be add to the polymer or a polymerization reaction to modify toincrease the cross-linking of the polymer.

As used herein, the term “template” refers to as a structure directingagent and is stable under hydrothermal aging conditions and furthermorehydrophobic relative to the metal salts. Many templates used in themanufacturing nanoparticles are known in the art. They include all typesof surfactants including anionic surfactants, cationic surfactants, andneutral surfactants as well as polymers with surfactant properties suchas Poloxamer. The surfactant may act as a nucleation site for theformation of the nanoparticles. Alkyl ammonium salts are commonly usedas templates to form structures in solution that interacts with theinorganic material in solution and serve as a template for the growth ofnanoparticles. A commonly used template is tetrapropylammoniumhydroxide, tetraethylammonium hydroxide, tetrabutylammonium hydroxide,or tetrapentylammonium hydroxide. Other known templates include cetyltrimethylammonium bromide, cetyl triethylammonium bromide, dodecyltriethylammonium bromide, Brij-56, or Brij-30. The template is usuallydecomposed during calcining at temperatures in the range 545-600° C. for6-10 hours.

In this disclosure, the terms “poloxamer” and “Pluronic” have the samemeaning and are used interchangeably. Poloxamers or Pluronics arenonionic triblock copolymers composed of a central hydrophobic block ofpolypropylene glycol flanked by two hydrophilic blocks of polyethyleneglycol (PEG) having the formula:

wherein “a” is any value in the range 2-130 and “b” is any value in therange 15 and 67. Poloxamers are also known by their trade nameSynperonicsTm, Pluronicslm and Kolliphorrm. Because the lengths of thepolymer blocks may be customized, many different poloxamers exist havingslightly different properties. For the generic term “poloxamer”, thesecopolymers are commonly named with the letter “P” (for poloxamer)followed by three digits: the first two digits multiplied by IN give theapproximate molecular mass of the polyoxypropylene core, and the lastdigit multiplied by 10 gives the percentage polyoxyethylene content,e.g. P407=poloxamer with a polyoxypropylene molecular mass of 4,000g/mole and a 70% polyoxyethylene content. For the Pluronic andSynperonic tradenames, coding of these copolymers starts with a letterto define its physical form at room temperature (L=liquid, P=paste,F=flake (solid)) followed by two or three digits, The first digit (twodigits in a three-digit number) in the numerical designation, multipliedby 300, indicates the approximate molecular weight of the hydrophobe;and the last digit×10 gives the percentage polyoxyethylene content(e.g., L61 indicates a polyoxypropylene molecular mass of 1,800 g/moland a 10% polyoxyethylene content). In the example given, poloxamer 181(P181)=Pluronic L61 and Synperonic PE/L 61. Pluronic F127, also known aspoloxamer 407, is a hydrophilic non-ionic surfactant wherein “a” and “b”are approximately block length of 101 and 56, respectively. PluronicP123 has the same chemical formula as that of Pluronic F127 except thata=20 and b=70. Many commercially available Pluronics including but notlimited to are Pluronic P108, Pluronic P103, Pluronic P104, and PluronicP105.

Multi-walled carbon nanotubes (MWCNT) consist of multiple concentricrolled layers of graphene tubes. There are two models that can be usedto describe the structures of MWCNT. In the Russian Doll model, sheetsof graphite are arranged in concentric cylinders, for example, asingle-walled nanotube within a larger single-walled nanotube. In theParchment model, a single sheet of graphite is rolled in around itself,resembling a scroll of parchment or a rolled newspaper. The interlayerdistance in MWCNT is close to the distance between graphene layers ingraphite of approximately 3.4 Å. The Russian Doll structure is observedmore commonly.

A first aspect of the invention is directed to a method of preparingMWCNT-ZrO₂ nanocomposite. The method comprises preparing a mixture bysuspending a zirconium alkoxide and MWCNT in an alcohol containing atemplating agent and sonicating. While agitating and heating themixture, a solution in the range of 1%-10%, preferably 1%-6%, morepreferably 1%-4% and most preferably 2%-3% acetic acid in water is addeddropwise until a precipitate is formed. The mixture may be heated at atemperature in the range 40-80° C., preferably in the range 50-70° C.,and more preferably in the range 55-60° C.

Any zirconium alkoxide may be utilized in the method including but notlimited to zirconium methoxide, zirconium ethoxide, zirconium propoxide,zirconium isopropoxide, zirconium butoxide, zirconium isobutoxide, andzirconium t-butoxide. Similarly, any solvent or combination of solventsmay be used in the method. In some embodiment, the solvent is misciblewith water including but not limited to acetonitrile, dimethylsulfoxide, dimethyl-formamide, and alcohols with alkyl group having 1-6carbon atoms. In some more preferred embodiments, the solvent is analcohol including methanol, ethanol, propanol, isopropanol, butanol,isobutanol, and t-butanol as well as pentanol and hexanol and isomersthereof. In the most preferred embodiment, the alcohol is methanol,ethanol, propanol, and/or isopropanol. In some other preferredembodiments, the alcohol and zirconium alkoxide have the same alkylgroup. For example, zirconium isopropoxide and isopropanol are usedtogether.

The precipitate may be separated from the mixture by filtration,decantation, or centrifugation and washed several times with water andlow boiling point alcohols such as methanol or ethanol. The resultingpowder may be dried in vacuum at a temperature in the range 50-90° C.,preferably in the range of 55-75° C., more preferably in the range of60-70° C. Following the drying, the powder may be calcined at atemperature in the range of 200-700° C., more preferably in the range of250-600° C., and most preferably in the range of 300-500° C.

A second aspect of the invention is the MWCNT-ZrO₂ nanocompositeprepared by the method described herein.

In some embodiments, the MWCNT-ZrO₂ nanocomposite contains MWCNT of theinvention in the range from 0.1 wt. % to 8.0 wt. %, preferably 0.5 wt. %to 6.0 wt. %, more preferably 1.0 wt. % to 5.0 wt. %, and mostpreferably 2.0 wt. % to 3 wt. % relative to the total weight of theMWCNT-ZrO₂ nanocomposite. In particular preferred embodiment, theMWCNT-ZrO₂ nanocomposite contains MWCNT in an amount selected from thegroup consisting of 1%, 2%, 3%, and 5% relative to the total weight ofthe MWCNT-ZrO₂ nanocomposite.

In some embodiments, the MWCNT-ZrO₂nanocomposite of the invention mayhave a surface area is in the range of 51.0±2 to 65.0±2 m²/g, morepreferably the surface area is in the range of 53.0±2 to 60,0±2 m²/g,and most preferably 54.0±2 to 58.0±2 m²/g. In a particular preferredembodiment of the invention, the surface area is 59.2±2 m²/g.

Another preferred embodiment, the MWCNT-ZrO₂nanocomposite of theinvention is a photocatalyst with a band gap ranging from 3.35 eV to2.50 eV, preferably ranging from 3.0 eV to 2.60 eV, more preferablyranging from 2.80 eV to 2.60 eV. In particularly preferred embodiment,the MWCNT-ZrO₂ nanocomposite has a band gap selected from the groupconsisting of about 2.81 eV, 2.70 eV, 2.69 eV, and 2.66 eV.

A third aspect of the invention is related to a photochemical method ofproducing hydrogen gas from water. The method comprises suspendingMWCNT-ZrO₂ nanocomposite in alcohol aqueous solution to form acomposition and irradiating the composition with light source andcollecting the evolved hydrogen. The light source may be any lightsource including, but not limited to a lamp such as tungsten lamp, xenonlamp or fluorescent lamp, or sun light. The method may requiredissipating the heat produced by the lamp or the sun. Thus, the methodmay utilize a cooling system such as circulating water bath and the likeor adding ice.

Any water miscible alcohol such as but not limited to methanol, ethanol,propanol, and isopropanol may be used. The amount of alcohol in theaqueous solution may be in the range of 1% to 30% (v/v), preferably inthe range of 5% to 20% (v/v), more preferably in the range of 8% to 15%(v/v), most preferably in the range of 9% to 12% (v/v). In a particularpreferred embodiment, the amount of alcohol is 10% (v/v). The water usedin the method may be treated or untreated sea water, river water, orunderground water. It may be filtered to remove solid materials such assand, clay, and other solids or biological materials.

In a preferred embodiment of the method, the amount of photocatalystpresent in the aqueous solution is in the range from 0.5 g/L to 3.0 g/L,more preferably in the range of 1.0 g/L to 2.5 g/L, and most preferablyin the range of 1.5 g/L to 2.5 g/L. In a particularly preferredembodiment, the amount of photocatalyst added to the aqueous solution is2.0 g/L. The photocatalyst is stable and may be recycled as long as itdisplays catalytic activity of at least 50%, preferably at least 70%,more preferably at least 80%, and most preferably at least 90% of thecatalytic activity observed in the first use. In a preferred embodiment,the photo catalyst may be recycled in the range 2 to 10 times, morepreferably in the range 3-500, and most preferably in the range500-10,000.

Any of the MWCNT-ZrO₂ nanocomposites of the invention may be utilized ina method to produce hydrogen. In a preferred embodiment of the method,the photocatalyst comprises MWCNT in an amount of 1%, 2%, 3%, 4%, and 5%relative to the total weight of the MWCNT-ZrO₂ nanocomposite.

The examples below are intended to further illustrate protocols for thepreparation and characteristics of the MWCNT-ZrO₂ nanocompositedescribed above, and are not intended to limit the scope of the claims.

EXAMPLE 1 Materials and Methods:

Transmission electron microscopy (TEM) of suspension of nanocomposite inmethanol of the nanocomposites was carried out by JEOL-JEM-1230. A Nova2000 series Chromatech was used to observe the surface area of thecomposite from N₂-adsorption desorption measurements. Crystalline phaseof the composites was determined by Bruker axis D8 with Cu Kα radiation(λ=1.540 Å) at room temperature. The band gap was calculated fromdiffuse reflectance spectra (UV-Vis-DRS). Photoluminescence emissionspectra (PL) were recorded on a Shimadzu RF-5301 fluorescencespectrophotometer. Transient photocurrent was recorded byelectrochemical workstation, Zahner Zennium, Germany.

EXAMPLE 2

Preparation of ZrO₂ nanoparticles: Solution A was prepared by dispersingzirconium isopropoxide in isopropyl alcohol and Pluronic F127 for 1 husing ultrasonic cleaner bath. The ratio of zirconium isopropoxide toisopropyl alcohol to Pluronic F127 was kept at 1:10: 0.3. Solution B wasprepared by adding 100 ml of deionized water to 2 ml of acetic acid.Solution B was added dropwise to solution A while stirring for 1 h at333° K until a precipitate is formed. The obtained powder was separatedby filtration, washed several times with deionized water and ethanol,and dried under vacuum at 333° K for 10 h. Finally, the powder wascalcined for 3 h under nitrogen gas at 573 K.

EXAMPLE 3

Preparation of MWCNT-ZrO₂ nanocomposites: In a ratio of 1:10:0.3(w/w/w), zirconium isopropoxide was dispersed in isopropyl alcohol andPluronic F127 for 1 h using ultrasonic cleaner bath. For 1% preparation,0.24 g of MWCNTs was added to 100 mL of the dispersion and sonicated foranother 3 h to form solution A. Solution B was prepared by adding 100 mlof deionized water to 2 ml of acetic acid. Solution B was added dropwiseto solution A while stirring for 1 h at 333° K (30° C.) until aprecipitate was formed. The precipitate was separated by filtration,washed several times with deionized water, and ethanol, and dried undervacuum at 333° K (30° C.) for 10 h. Finally, the powder was calcined for3 h under nitrogen gas at 573° K (60° C.). The same method was used toprepare 2 wt. % MWCNT-ZrO₂, 3 wt. % MWCNT-ZrO₂, 4 wt. % MWCNT-ZrO₂ and 5wt. % MWCNT-ZrO₂ by using the appropriate amount of MWCT.

EXAMPLE 4

Hydrogen production application: Hydrogen production experiments werecarried out using ZrO₂, 1, 2, 3, 4, and 5 wt. % MWCNT-ZrO₂nanocomposites. In a typical experiment, 2 g/L of catalyst was added to450 mL aqueous solution (10 vol. % methanol). The reaction system wasequipped with quartz cooler to dissipate the heat generated by the 500WXenon lamp. The lamp irradiated the reaction mixture while stirring andthe evolved hydrogen produced was quantified by Agilent GC 7890A system.Also, control experiments containing the reaction mixture without thephotocatalyst were carried out.

EXAMPLE 5 Characterizations of the Nanoparticles:

The XRD patterns of MWCNT, ZrO₂, and MWCNT-ZrO₂ containing 1-5% MWCNTare shown in FIG. 1. The patterns display pure ZrO₂ phase in all sixsamples without peaks of MWCNT due to the low content of MWCNT in thenanocomposites. Noticeable intensity decrease of ZrO₂ is observed as thewt. % of MWCNT increases indicating a decrease of crystallite sizes ofZrO₂ nanoparticles.

The morphology of ZrO₂, MWCNT, 1 wt. % MWCNT-ZrO₂, 2 wt. % MWCNT-ZrO₂, 3wt. % MWCNT-ZrO₂, 4 wt. % MWCNT-ZrO₂ and 5 wt. % MWCNT-ZrO₂ is examinedby TEM. FIG. 2 shows TEM images of MWCNT, ZrO₂, and MWCNT-ZrO₂containing 1 wt. %-5 wt. %. The particle size of ZrO₂ is small anduniform as shown in FIG. 2(A). The diameter of pure MWCNT is about 20 nmas shown in Fig.2 B. Also, a uniform cover of MWCNT by ZrO₂ is observedas shown in FIG. 2 (C to E). Increase of the weight percent of MWCNTabove 3 wt. % leads to agglomeration of ZrO₂ as shown in FIG. 2 (F andG).

BET surface area values of ZrO₂, MWCNT, 1 wt. % MWCNT-ZrO₂, 2 wt. %MWCNT-ZrO₂, 3 wt. % MWCNT-ZrO₂, 4 wt. % MWCNT-ZrO₂ and 5 wt. %MWCNT-ZrO₂ samples are shown in Table 1. The increase of the amount ofMWCNT from 0 to 3 wt. % improves the BET surface area of ZrO₂ from 50m²/g to 59.2 m²/g. Above 3 wt. %, increasing MWCNT has no significanteffect on BET surface area.

TABLE 1 BET surface area of ZrO₂ and MWCNT-ZrO₂ nanocomposites. SampleBET surface area. M²/g ZrO₂ 50.00 1 wt. % MWCNT-ZrO₂ 53.50 2 wt. %MWCNT-ZrO₂ 56.10 3 wt. % MWCNT-ZrO₂ 59.20 4 wt. % MWCNT-ZrO₂ 59.30 5 wt.% MWCNT-ZrO₂ 59.50

FIG. 3 shows the UV/vis spectra of MWCNT-ZrO₂. The results in FIG. 3show a redshift of λ_(max) as the amount of MWCNT in the nanocompositeincreases. Also, the band gap narrows as the amount of MWCNT in thenanocomposite increases. For example the increase of the percentage ofMWCNT from 0 to 3 wt. % decreases band gap energy of ZrO₂ from 3.32 to2.67 eV. Above 3 wt. % increase the band gap narrowing is unnoticeable.

Photoluminescence emission spectra (PL) of ZrO₂ and 1-5 wt % MWCNT-ZrO₂nanocomposites (FIG. 4) show the peak intensity decrease in thefollowing order ZrO₂>1 wt % MWCNT-ZrO₂>2 wt % MWCNT-ZrO₂>5 wt %MWCNT-ZrO₂>4 wt % MWCNT-ZrO₂>3 wt % MWCNT-ZrO₂. The observed red shiftis a result of the dispersion of ZrO₂ on the surface of MWCNT. Thevalues of band gap energy of ZrO₂, 1 wt % MWCNT-ZrO₂, 2 wt % MWCNT-ZrO₂,3 wt % MWCNT-ZrO₂, 4 wt % MWCNT-ZrO₂ and 5 wt % MWCNT-ZrO₂ arecalculated from their PL emission spectra are 3.31±0.15, 2.94±0.15,2.81.94±0.14, 2.66±0.13, 2.69±0.14 and 2.70 eV±0.14, respectivelyconfirming data observed from the UV-Vis spectra.

Transient photocurrent responses of ZrO₂ and 3 wt % MWCNT-ZrO₂photocatalyst are shown in FIG. 5. The result shows that 3 wt %MWCNT-ZrO₂ has 7 times greater photocurrent density than that of ZrO₂indicating that the rate of electron-hole recombination is very smallfor the 3 wt % MWCNT-ZrO₂ as compared to that of ZrO₂.

Performance of the Nanocomposite in the H₂ Evolution Experiment:

FIG. 6 shows time course for the hydrogen produced by the photochemicalreactions catalyzed by 2 g/L of ZrO₂, 1 wt. % MWCNT-ZrO₂, 2 wt. %MWCNT-ZrO₂′ 3 wt. % MWCNT-ZrO₂, 4 wt. % MWCNT-ZrO₂, and 5 wt. %MWCNT-ZrO₂ of nanocomposites MWCNT-ZrO₂ using 500W xenon lamp. As shownin FIG. 6, ZrO₂ has very little or no photocatalytic activity becauseZrO₂ absorbs in the UV region and the reaction mixture is irradiatedwith visible light. The decrease in the band gap upon the addition ofMWCNT to ZrO₂ leads to large increase in the catalytic efficiency of thezirconium catalyst. The hydrogen evolution for the photochemicalreaction-catalysed by ZrO₂ is 31.2 μmol compared to 2000 μmol for thatcatalysed by 3 wt. % MWCNT-ZrO₂. FIG. 7 shows the time course forhydrogen evolution from the photochemical reaction-catalyzed by anamount of 3 wt. % MWCNT-ZrO₂ ranging from 0.5 g/L to 3.0 g/L. Asexpected, the amount of hydrogen evolved in the photochemical reactionis increased with increasing the amount of catalyst up to 2.0 g/L due toincrease in number of available sites for the photocatalytic reaction.Above 2 g/L catalyst, the amount of hydrogen evolved decreases withincreasing the amount of catalyst probably due to the increased amountof catalyst hinders the light penetration in the reaction mixture.

Recycling and reuse of 3 wt. % MWCNT-ZrO₂ photocatalyst on amount ofhydrogen evaluation is examined using 2.0 g/L of 3 wt. % MWCNT-ZrO₂photocatalyst in 450 mL aqueous solution irradiated with 500W xenon lamp9 h. FIG. 8 shows recycling and reusing of 3 wt. % MWCNT-ZrO₂photocatalyst on amount of hydrogen evaluation. It is clear that 3 wt. %MWCNT-ZrO₂ photocatalyst has photocatalytic stability and can be usedand recycled many times without significant loss of catalytic activity.

The present disclosure describes a method for the synthesis of ZrO₂nanoparticles and MWCNT-ZrO₂ nanocomposites in the presence of apoloxamer template at room temperature. The photocatalytic activity of 3wt. % MWCNT-ZrO₂ nanocomposites is significantly better photocatalystthan ZrO₂, 1 wt. % MWCNT-ZrO₂, 2 wt. % MWCNT-ZrO₂, 4 wt. % MWCNT-ZrO₂and 5 wt. % MWCNT-ZrO₂ for hydrogen production by 64.1, 3.2, 1.6, 1.2and 1.1 times, respectively. The photocatalyst 3 wt. % MWCNT-ZrO₂displays photocatalytic stability and can be recycled in a method forthe production of hydrogen from water.

1. An alcohol-based method of making a ZrO₂-multi-walled carbon nanotubenanocomposite, comprising: dispersing, while sonicating, a zirconiumalkoxide and a poloxamer in an alcohol to form a mixture, wherein theratio of the zirconium alkoxide to the alcohol to the poloxamer is inthe range of 0.2:10:0.1 to 3:10:0.5 (w/w/w), mixing a multi-walledcarbon nanotube (MWCNT) with the mixture to form a MWCNT-containingmixture, and mixing an aqueous carboxylic acid solution with theMWCNT-containing mixture to precipitate the ZrO₂-multi-walled carbonnanotube nanocomposite precursor, then calcining the ZrO₂-multi-walledcarbon nanotube nanocomposite precursor under nitrogen to form theZrO₂-multi-walled carbon nanotube nanocomposite; wherein theZrO₂-multi-walled carbon nanotube nanocomposite comprises 0.5% -5% byweight of the MWCNT.
 2. (canceled)
 3. (canceled)
 4. The method of claim1, wherein the zirconium alkoxide is zirconium isopropoxide and thealcohol is isopropyl alcohol.
 5. (canceled)
 6. The method of claim 1,wherein the carboxylic acid is acetic acid.
 7. (canceled)
 8. (canceled)9. The method of claim 1, wherein the calcining is at about 300° C.under nitrogen. 10-20. (canceled)